Signal processing devices have been suggested by the prior art for providing for the processing and storage of signals by utilizing a piezoelectric substrate capable of propagating acoustic wave signals on a selected surface thereof and a semiconductor substrate positioned adjacent and spaced from such surface. Appropriate techniques are utilized for altering the conductivity pattern in the semiconductor substrate in accordance with the wave form of an acoustic wave signal that is propagated along the selected surface of the piezoelectric substrate so that a representation of the acoustic wave signal is effectively and temporarily stored therein. Such techniques for altering the conductivity pattern include applying a signal uniformly over the interaction region which comprises the regions at or near the surfaces of the substrates and the spatial region therebetween so that a second signal which is propagated along the surface of the piezoelectric material interacts with the uniformly applied signal to alter the conductivity pattern in the semiconductor substrate, the altered conductivity pattern representing the stored propagated signal. A further signal subsequently propagated along the piezoelectric substrate surface thereupon interacts with the stored altered conductivity pattern, the interaction thereby producing an output signal at an electrode of the semiconductor substrate which represents the correlation or the convolution of the two interacting signals depending on the direction of propagation of the further signal along the piezoelectric surface. Certain structural embodiments of such technique have been discussed in the articles of Stern and Williamson, "New Adaptive-Signal-Processing Concept" in Electronic Letters, Vol. 10, No. 5, dated Mar. 7, 1974, and of Bers and Cafarella, "Surface State Memory in Surface Acoustoelectric Correlator", Applied Physics Letters, Vol. 25, No. 3, dated Aug. 1, 1974, and in the copending applications, Ser. No. 555,367 of Stern et al., filed on Mar. 5, 1975 and now U.S. Pat. No. 4,016,412, Ser. No. 672,345 of Stern et al., filed on Mar. 31, 1976 and now U.S. Pat. No. 4,075,706, and Ser. No. 672,344 of Stern et al., filed on Mar. 31, 1976.
The major problems with such devices have been that a relatively long time period is required in order to store a signal in the form of such altered stationary conductivity pattern in the previously disclosed embodiments and, once stored, the signal can remain stored therein only for a relatively short time period. In utilizing trap techniques, as disclosed in the above Bers et al. article and the patent applications, for example, the time required to store a signal may be in the order of 0.1 to 1 microseconds (.mu. sec.), while the signal can be held in storage only for about 0.1 to 1 milliseconds (msec.), or less. The usefulness of such devices thereby becomes limited because of the relatively long storing or "write" time period and the relatively short storage time period. It is desirable, therefore, to improve such techniques by reducing the write time periods and increasing the storage times.